The present invention relates to devices for electric circuitry. More precisely, the invention relates to low voltage capacitors that produce minimal inductance, e.g., for snubber circuitry, and to methods for minimizing inductance in same.
Capacitance, i.e., the ability of capacitors to store charge, plays a dominant role in shaping the time and frequency response of modern electrical circuitry. However, capacitance can be affected deleteriously by stray inductance, which is a flux field that is produced by current flowing through wires and/or conduction paths of substantially all electrical circuits and circuitry. Indeed, current (I) flowing through circuitry can cause significant voltages (V) to develop across stray inductance (L), which condition diminishes capacitance (C). Therefore, for optimal performance, inductance associated with capacitors must be as low as possible. This is especially true at higher frequencies, i.e., 5 kHz and higher, where inductive impedance diminishes capacitive effectiveness by amplifying microsecond pulses to high voltage levels.
This is particularly true of snubber capacitors, which are connected in parallel with one or more switches as part of a snubber circuit. Snubber circuits typically are used to (i) reduce voltage spikes, e.g., switching pulses, that occur when power switches are turned on and off, i.e., xe2x80x9cswitchedxe2x80x9d, repeatedly and (ii) to reduce power and other losses that also result from repeated switching. Indeed, power loss, which is defined by the equation P=Vxc3x97I, is substantially zero when either there is no current flow, i.e., I=0, which occurs when the switch is off, and/or when the voltage is very low, i.e., V= greater than 0. When neither of these conditions exists, the circuitry experiences significant power loss.
As a result, snubber capacitors, which typically operate at high current levels, i.e., I greater than  greater than 0, by default, must be designed to suppress switching pulses in inverter circuits to as low voltage levels as possible. Accordingly, the use of snubber capacitors in this capacity requires a very low inductance, otherwise, solid state devices, e.g., insulated gate bi-polar transistors (xe2x80x9cIGBTsxe2x80x9d), and/or insulation may fail if the inductance and associated voltage are too high.
Bowers (U.S. Pat. No. 6,166,932) discloses snubber circuitry that minimizes oscillations that are caused by repeated switching. Indeed, the snubber circuitry of Bowers includes a pair switches in parallel with a pair of capacitors, wherein an air-core transformer that is in series with a relatively high value resistor is disposed in close proximity of the capacitors to inductively couple resistance in the branch circuit without introducing additional stray inductance, which might otherwise be the case were the resistors directly connected to the capacitors in series. Hence, Bowers purports to reduce inductance in the snubber circuitry by providing distance between the resistors and the capacitors.
Furthermore, the prior art keeps inductance in interconnects and terminals by making interconnects and terminals as short as possible and/or by reducing the distance between adjacent edges of terminals.
However, the prior art has apparently failed to appreciate that to obtain low inductance in snubber capacitor design: (i) the cross-sectional area of the capacitor should be as large as possible, while the length, however, is kept to a minimum; (ii) design includes a high aspect ratio for low cross-sectional charge density; and (iii) conductors and/or terminals of opposite polarity should be disposed as close to one another as possible so that their respective flux fieldsxe2x80x94which produce inductancexe2x80x94substantially cancel out each other.
The present invention produces a low-inductance capacitance device, which is particularly useful with snubber circuitry. The cross-sectional area of the disclosed capacitor is relatively large although its length is kept to a minimum. The present invention provides a capacitor that includes a high aspect ratio for low cross-sectional charge density. Furthermore, the present invention comprises capacitors in combination with conductors and/or terminals of opposite polarity that are disposed close to one another so that the inductance produced by the flux fields substantially cancels out each other.
Accordingly, the invention provides a low-inductance device for storing electrical charge comprising a plurality of extended electrodes, wherein electrical current passing through a extended electrode from said plurality of extended electrodes flows in a direction opposite of current passing through one or more adjacent extended electrodes of said plurality of extended electrodes to provide cancellation of inductance in the device.
The invention as provides a method of reducing the inductance of a device used for storing an electrical charge, the device having a plurality of extended electrodes having a first end spray and a second end spray, the method comprising the steps of: sandwiching an insulating film between a positive conductor terminal and a negative conductor terminal; and joining said negative conductor terminal to each of the plurality of extended electrodes at a first end spray and said positive conductor terminal to each of the plurality of extended electrodes at a second end spray so that the end sprays of adjacent extended electrodes are joined to the negative conductor terminal and the positive conductor terminal alternately so that current flows in opposite directions between first and second end sprays of adjacent extended electrodes.